The assembly of electrical, optical, and mechanical devices is trending toward smaller dimensions and increased sensitivity to thermal and mechanical stresses during high-speed assembly processes. After assembly, these devices must often be protected from ambient atmospheres, in addition to continued protection from thermal and mechanical stresses. Room temperature vulcanizing (RTV) silicones have traditionally played a significant role in component protection. Desirable performance improvements of silicone RTVs, which are particularly significant at smaller geometries, would include viscosity reduction of the silicone prepolymer, rapid cure at low temperatures and increased mechanical properties.
Room temperature vulcanizing silicone elastomers are documented in a significant amount of literature. (See, for example, E. Warrick et al., Rubber Chem. Tech., 52(3):437 (1979) and O. Dolgov et al., “Organosilicon Liquid Rubbers,” Int'l Poly. Sci. & Techn., Monograph #1, RAPRA (1975)). More specifically, vinyl-addition silicone RTVs typically cure by a zero valent platinum-catalyzed crosslinking reaction between a telechelic vinyl-functional silicone and a methylhydrosiloxane-dimethylsiloxane copolymer.
The prior art provides no examples of silicone elastomers which are essentially linear structures derived from low molecular weight precursors. This may not be surprising when considering that linear polydimethysiloxanes of molecular weights greater than 400,000 Daltons still manifest liquid behavior, albeit with viscosities greater than 10,000,000 cSt, rather than elastomeric behavior. Most commercial silicones are produced by equilibrium polymerization and polydisperse (PDIs (Mw/Mn) at high molecular weights approaching 3) and >5% residual low molecular weight cyclic and linear siloxanes.
Accordingly, single monomer step-growth polymerization as a strategy to form silicone elastomers has not been pursued. “Single monomer step-growth polymerization” refers to a type of polymerization mechanism in which dual-functional monomers react to first form dimers, then trimers, and eventually long chain polymers. Synthetic polymers produced by step-growth polymerization include polyamides and polyurethanes. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight.
Monovinyl terminated siloxanes prepared by living anionic ring-opening polymerization (AROP) are described in DE 44 36 076, but the fact that these polymers are terminated with alkoxysilanes does not allow for the possibility of step growth polymerization. Shintani et al., Polymer Bulletin, 37:705-710 (1996) describes the synthesis of a series of dual functional monomers and oligomers by degradative reaction of organolithium reagents with cyclic siloxanes and quenching with a chlorosilane. The term “oligomer” may be understood to mean a molecule containing a few monomer units (oλιγoζ, or oligos, is Greek for “a few”), nominally in a range of 2-6. Monovinyl, monohydride terminated oligomers with 3 monomer units (4 silicon atoms) were prepared by Shintani et al. and self-reacted in the presence of a variety of catalysts.

The highest Mn achieved by Shintani was 5400 Daltons, which corresponds to a slightly viscous liquid having a viscosity of 50-100 cSt. The inability to achieve high molecular weights may be due to the fact that the low molecular weight materials have a significant probability of forming cyclic species, especially low molecular weight cyclic species. Alternatively or additionally, loss of even a small portion of hydride termination, perhaps due to oxidation or reaction with water, results in the conversion of these monomers to chain termination species. It may also be possible that Shintani was not able to achieve the high levels of purity necessary for step-growth polymerization because his yield of monomer was low, possibly indicative of reactive byproducts. Effective step-growth polymerization requires an exact 1:1 ratio of vinyl and hydride substitution within one molecule, without any impurity or byproduct reaction that would inhibit or eliminate the ability to form high molecular weight polymers.
Two monomer step-growth polymerization of silicones is a methodology in which a telechelic hydride terminated siloxane is reacted with a telechelic vinyl terminated siloxane in the presence of a Pt catalyst, as exemplified by M. Grunlan et al., Polymer, 44:981 (2003). These systems result in chain extension polymers that are typically highly disperse. Grunlan describes molecular weights in the range of 14,000 to 20,000 with PDIs in the range of 1.5 to 3.
In any case, there is little precedent to expect polydimethylsiloxanes of high molecular weight to exhibit elastomeric properties. Non-crosslinked silicones having molecular weights exceeding 500,000 are commonly available and referred to as silicone gums. Unless crosslinked, gums do not behave as elastomers.